Photodetectors including a coupling region with multiple tapers

ABSTRACT

Structures for a photodetector and methods of fabricating a structure for a photodetector. A photodetector includes a photodetector pad coupled to a waveguide core and a light-absorbing layer coupled to the photodetector pad. The light-absorbing layer has a body, a first taper that projects laterally from the body toward the waveguide core, and a second taper that projects laterally from the body toward the waveguide core. The photodetector pad includes a tapered section that is laterally positioned between the first taper and the second taper of the light-absorbing layer.

BACKGROUND

The present invention relates to photonics chips and, more specifically,to structures for a photodetector and methods of forming a structure fora photodetector.

Photonics chips are used in many applications and systems including, butnot limited to, data communication systems and data computation systems.A photonics chip integrates optical components, such as waveguides,optical switches, directional couplers, and bends, and electroniccomponents, such as field-effect transistors, into a unified platform.Among other factors, layout area, cost, and operational overhead may bereduced by the integration of both types of components on the same chip.

Photonics chips may include photodetectors that convert modulated pulsesof light into an electrical signal. Photodetectors may suffer fromsignificant back reflection producing optical return loss.Photodetectors may also exhibit significant responsivity degradationover time.

Improved structures for a photodetector and methods of fabricating astructure for a photodetector are needed.

SUMMARY

In an embodiment of the invention, a structure includes a waveguide coreand a photodetector having a photodetector pad coupled to the waveguidecore. The photodetector further includes a light-absorbing layer coupledto the photodetector pad. The light-absorbing layer has a body, a firsttaper that projects laterally from the body toward the waveguide core,and a second taper that projects laterally from the body toward thewaveguide core. The photodetector pad includes a tapered section that islaterally positioned between the first taper and the second taper of thelight-absorbing layer.

In an embodiment of the invention, a method includes forming a waveguidecore and a photodetector pad coupled to the waveguide core, and forminga light-absorbing layer coupled to the photodetector pad. Thelight-absorbing layer includes a body, a first taper that projectslaterally from the body toward the waveguide core, and a second taperthat projects laterally from the body toward the waveguide core. Thephotodetector pad includes a tapered section that is laterallypositioned between the first taper and the second taper of thelight-absorbing layer. The light-absorbing layer and the photodetectorpad are included in a photodetector.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate various embodiments of theinvention and, together with a general description of the inventiongiven above and the detailed description of the embodiments given below,serve to explain the embodiments of the invention. In the drawings, likereference numerals refer to like features in the various views.

FIG. 1 is a diagrammatic top view of a structure at an initialfabrication stage of a processing method in accordance with embodimentsof the invention.

FIG. 2 is a cross-sectional view taken generally along line 2-2 in FIG.1.

FIG. 3 is a top view of the structure at a fabrication stage of theprocessing method subsequent to FIG. 1.

FIG. 4 is a cross-sectional view taken generally along line 4-4 in FIG.3.

FIG. 4A is a cross-sectional view taken generally along line 4A-4A inFIG. 3.

FIG. 5 is a top view of the structure at a fabrication stage of theprocessing method subsequent to FIG. 1.

FIG. 6 is a cross-sectional view taken generally along line 6-6 in FIG.5.

FIG. 6A is a cross-sectional view taken generally along line 6A-6A inFIG. 5.

FIGS. 7, 7A are cross-sectional views at a fabrication stage of theprocessing method subsequent to FIGS. 6, 6A.

FIG. 8 is a top view of a structure in accordance with alternativeembodiments of the invention.

FIG. 9 is a cross-sectional view taken generally along line 9-9 in FIG.8.

FIG. 9A is a cross-sectional view taken generally along line 9A-9A inFIG. 8.

FIG. 10 is a top view of a structure in accordance with alternativeembodiments of the invention.

FIG. 11 is a cross-sectional view taken generally along line 11-11 inFIG. 10.

FIG. 11A is a cross-sectional view taken generally along line 11A-11A inFIG. 10.

FIG. 12 is a cross-sectional view of a structure in accordance withalternative embodiments of the invention.

FIG. 13 is a top view of a structure in accordance with alternativeembodiments of the invention.

DETAILED DESCRIPTION

With reference to FIGS. 1, 2 and in accordance with embodiments of theinvention, a structure 10 may be formed using asemiconductor-on-insulator (SOI) substrate that includes a device layer12, a dielectric layer 14, and a handle wafer 16. The device layer 12 isseparated from the handle wafer 16 by the intervening dielectric layer14 and may be significantly thinner than the handle wafer 16. The devicelayer 12 and the handle wafer 16 may contain a single-crystalsemiconductor material, such as single-crystal silicon, and may belightly doped to have, for example, p-type conductivity. The dielectriclayer 14 may be comprised of a dielectric material that provideslow-index cladding and, in an embodiment, may be a buried oxide layercontaining silicon dioxide.

A waveguide core 18 and a photodetector pad 20 connected to a portion ofthe waveguide core 18 are defined by patterning the device layer 12. Theshapes of the waveguide core 18 and photodetector pad 20 may be definedby patterning trenches in the device layer 12 with lithography andetching processes, depositing a dielectric material (e.g., silicondioxide) in the trenches to form shallow trench isolation regions 24,and planarizing with chemical-mechanical polishing. The shallow trenchisolation regions 24 may penetrate fully through the device layer 12 tothe dielectric layer 14.

The waveguide core 18 and photodetector pad 20 are arranged withalignment along a longitudinal axis 22. The photodetector pad 20 has aside surface 21, the waveguide core 18 is coupled to a portion of theside surface 21 by a direct physical connection, and the waveguide core18 may be tapered with a width that increases with decreasing distancefrom the side surface 21.

With reference to FIGS. 3, 4, 4A in which like reference numerals referto like features in FIGS. 1, 2 and at a subsequent fabrication stage, adielectric layer 26 may be deposited and patterned by lithography andetching processes to define a window or opening having a shape for asubsequently-formed trench. The dielectric layer 26 may be composed of adielectric material, such as silicon nitride. The opening 25, whichpenetrates fully through the dielectric layer 26, exposes an area on thetop surface 19 of the photodetector pad 20. The exposed area is only aportion (i.e., a fraction) of the total area of the top surface 19 ofthe photodetector pad 20. The etching process may remove the material ofthe dielectric layer 26 selective to the material of the photodetectorpad 20. As used herein, the terms “selective” and “selectivity” inreference to a material removal process (e.g., etching) denote that thematerial removal rate (i.e., etch rate) for the targeted material ishigher than the material removal rate (i.e., etch rate) for at leastanother material exposed to the material removal.

A trench 28 is formed in the photodetector pad 20 and penetrates indepth partially through the photodetector pad 20. The trench 28 may beformed by an etching process, such as a reactive ion etching process.The patterned dielectric layer 26 functions as an etch mask during theetching process, and the opening 25 in the patterned dielectric layer 26defines the location of trench 28 on a top surface 19 of thephotodetector pad 20 and the shape for the trench 28. As a result, thetrench 28 is surrounded in a tub in the photodetector pad 20 and the tubis bounded by the semiconductor material of the device layer 12.Surfaces of the photodetector pad 20 are exposed at the bottom of thetrench 28 and at all sides of the trench 28.

The trench 28 includes a section 30, a section 32 that projectslaterally from the section 30 in a longitudinal direction toward thewaveguide core 18, and a section 33 that also projects laterally fromthe section 30 in a longitudinal direction toward the waveguide core 18.The sections 32, 33 of the trench 28 may have terminating regions thatare positioned adjacent to the side surface 21. The terminating regionsof the sections 32, 33 of the trench 28 may be spaced from the sidesurface 21 of the photodetector pad 20 by a gap, G, such that portionsof the semiconductor material of the photodetector pad 20 are positionedbetween the terminating regions of the sections 32, 33 and the sidesurface 21. In an alternative embodiment, the terminating regions of thesections 32, 33 of the trench 28 may intersect the side surface 21 suchthat a gap is absent. In embodiments, the distance between theterminating regions of the sections 32, 33 of the trench 28 and the sidesurface 21 may range from 0 microns to 1 micron. The trench 28 is shapedwith the sections 32, 33 projecting laterally as prongs in a directionalong the longitudinal axis 22 toward the waveguide core 18. Thewaveguide core 18 may be symmetrically arranged relative to the sections32, 33 with the section 32 on one side of the longitudinal axis 22, andwith the section 33 on an opposite side of the longitudinal axis 22. Thesections 32, 33 may have a shape that linearly varies over theirrespective lengths based on a linear function. In an alternativeembodiment, sections 32, 33 may have a shape that linearly varies overtheir respective lengths based on a non-linear function, such as aquadratic, parabolic, or exponential function.

With reference to FIGS. 5, 6, 6A in which like reference numerals referto like features in FIGS. 3, 4, 4A and at a subsequent fabricationstage, a light-absorbing layer 34 is formed inside the trench 28 in thephotodetector pad 20. In an embodiment, the light-absorbing layer 34 maybe formed by depositing a light-absorbing material with a chemical vapordeposition process in the trench 28. In an embodiment, thelight-absorbing layer 34 may be selectively deposited such that thelight-absorbing material deposits inside the trench 28 but does notdeposit on the dielectric layer 26. Subsequent to the formation of thelight-absorbing layer 34, the dielectric layer 26 may be removed by aselective etching process.

The light-absorbing layer 34 may define a light-absorbing region of aphotodetector, and light-absorbing layer 34 may be comprised of amaterial that generates charge carriers from the absorbed light. In anembodiment, the light-absorbing layer 34 may comprise a material havinga composition that includes germanium. In an embodiment, thelight-absorbing layer 34 may comprise a material having a compositionthat exclusively contains elemental germanium. The light-absorbing layer34 may be formed inside the trench 28 such that the light-absorbingmaterial is at least partially embedded in the photodetector pad 20. Inthe representative embodiment, the light-absorbing layer 34 includes aportion that is positioned above the top surface 19 of the photodetectorpad 20. In an alternative embodiment, the light-absorbing layer 34 maybe coplanar with the top surface 19 of the photodetector pad 20.

The light-absorbing layer 34 adapts and conforms to the shape of thetrench 28 when formed. In that regard, the light-absorbing layer 34includes a body 36, a taper 38 that projects laterally from the body 36in a longitudinal direction toward the waveguide core 18, and a taper 40that also projects laterally from the section 30 in a longitudinaldirection toward the waveguide core 18. The body 36, the taper 38, andthe taper 40 of the light-absorbing layer 34 are respectively positionedin the different corresponding sections 30, 32, 33 of the trench 28.

The tapers 38, 40 of the light-absorbing layer 34 may have non-pointed,blunt tips 35 that are positioned adjacent to the side surface 21 of thephotodetector pad 20. The tips 35 of the tapers 38, 40 may be spacedfrom the side surface 21 of the photodetector pad 20 by the gap, G, suchthat portions of the semiconductor material of the photodetector pad 20are positioned between the tips of the tapers 38, 40 and the sidesurface 21. In an alternative embodiment, the tapers 38, 40 of thelight-absorbing layer 34 may intersect the side surface 21 such that agap is absent. In embodiments, the distance between the tips 35 of thetapers 38, 40 and the side surface 21 may range from 0 microns to 1micron. The light-absorbing layer 34 reproduces the shape of the trench28 with the tapers 38, 40 projecting as prongs in a direction toward thewaveguide core 18.

A tapered section 31 of the photodetector pad 20 is positioned in thespace between the taper 38 and the taper 40. The waveguide core 18 maybe symmetrically arranged relative to the tapers 38, 40 with the taper38 on one side of the longitudinal axis 22 and tapered section 31, andwith the taper 40 on an opposite side of the longitudinal axis 22 andtapered section 31.

The tapers 38, 40 of the light-absorbing layer 34 and the taperedsection 31 of the photodetector pad 20 between the tapers 38, 40collectively define a coupling region 52 that optically couples thewaveguide core 18 with the light-absorbing layer 34. The coupling region52 is a composite of the material (e.g., germanium) of thelight-absorbing layer 34 in tapers 38, 40 and the different material(e.g., silicon) of the photodetector pad 20 in tapered section 31. Thelight-absorbing layer 34 has a side surface 42 and a side surface 44opposite to the side surface 42, and a side surface 46. The taperedsection 31 of the photodetector pad 20 has an interface with the sidesurface 46 over which the tapered section 31 and the side surface 46 arecontacting.

The distance, D1, between the side surface 42 of the taper 38 and theside surface 44 of the taper 40 in the coupling region 52 may increasewith decreasing distance from the waveguide core 18 and the side surface21 of the photodetector pad 20. In an embodiment, the distance, D1,distance between the side surface 42 of the taper 38 and the sidesurface 44 of the taper 40 in the coupling region 52 may be greater thanthe distance, D2, between the side surfaces 42, 44 in the body 36. Thewidth, W, of the tapered section 31 of the photodetector pad 20 mayincrease with increasing distance from the body 36. The width, W, of thetapered section 31 of the photodetector pad 20 may increase withdecreasing distance from the waveguide core 18 and the side surface 21of the photodetector pad 20

In alternative embodiments, other material combinations may be used toconstruct the structure 10. For example, the waveguide core 18 andphotodetector pad 20 be comprised of silicon nitride.

With reference to FIGS. 7, 7A in which like reference numerals refer tolike features in FIGS. 6, 6A and at a subsequent fabrication stage,doped regions 48, 50 are formed in respective portions of thephotodetector pad 20. The doped regions 48, 50 may extend through theentire thickness of the photodetector pad 20 to the underlyingdielectric layer 14. The doped region 48 and the doped region 50 may bearranged at the opposite side surfaces 42, 44 of the light-absorbinglayer 34 to define an anode and a cathode of a photodetector that alsoincludes the light-absorbing layer 34 and the photodetector pad 20. Thetapered section 31 of the photodetector pad 20 may be masked during theimplantations and not belong to either of the doped regions 48, 50.

The doped region 48 may be formed by, for example, ion implantation withan implantation mask with an opening that determines the implanted areaof the photodetector pad 20. The implantation mask may include a layerof a light-sensitive material, such as a photoresist, applied by aspin-coating process, pre-baked, exposed to light projected through aphotomask, baked after exposure, and developed with a chemical developerto define openings arranged over the areas to be implanted. Theimplantation conditions (e.g., ion species, dose, kinetic energy) may beselected to tune the electrical and physical characteristics of thedoped region 48. The implantation mask may be stripped after forming thedoped region 48. In an embodiment, the semiconductor material of thedoped region 48 may contain a p-type dopant (e.g., boron) that providesp-type electrical conductivity.

The doped region 50 may be formed by, for example, ion implantation withan implantation mask with an opening that determines the implanted areaof the photodetector pad 20. The implantation mask may include a layerof a light-sensitive material, such as a photoresist, applied by aspin-coating process, pre-baked, exposed to light projected through aphotomask, baked after exposure, and developed with a chemical developerto define openings arranged over the areas to be implanted. Theimplantation conditions (e.g., ion species, dose, kinetic energy) may beselected to tune the electrical and physical characteristics of thedoped region 50. The implantation mask may be stripped after forming thedoped region 50. In an embodiment, the semiconductor material of thedoped region 50 may contain an n-type dopant (e.g., phosphorus and/orarsenic) that provides n-type electrical conductivity.

Middle-of-line (MOL) processing and back-end-of-line (BEOL) processingfollow, which includes formation of silicide, contacts, vias, and wiringfor an interconnect structure that is coupled with the photodetectors.In particular, separate sets of contacts may be formed in a dielectriclayer of the interconnect structure that respectively extend to thedoped regions 48, 50.

In use, laser light may be guided by the waveguide core 18 to thelight-absorbing layer 34. The light-absorbing layer 34 absorbs photonsof the laser light and converts the absorbed photons into chargecarriers. The biasing of the doped regions 48, 50 causes the chargecarriers to be collected and output to provide, as a function of time, ameasurable photocurrent.

The coupling region 52 may be effective to improve the performance ofthe photodetector by, for example, suppressing back reflection. Thecoupling region 52 may also promote optical coupling of laser light tothe light-absorbing layer 34 by both butt-end and side evanescentcoupling mechanisms.

With reference to FIGS. 8, 9, 9A in which like reference numerals referto like features in FIGS. 5, 6, 6A and in accordance with alternativeembodiments, the distance, D1, between the side surface 42 of the taper38 and the side surface 44 of the taper 40 in the coupling region 52 maybe constant with decreasing distance from the waveguide core 18 and theside surface 21 of the photodetector pad 20. In an embodiment, thedistance, D1, between the side surface 42 of the taper 38 and the sidesurface 44 of the taper 40 in the coupling region 52 may be equal to thedistance, D2, between the side surfaces 42, 44 of the body 36.Independent of this modification to the tapers 38, 40, the width, W, ofthe tapered section 31 of the photodetector pad 20 decreases withincreasing distance from the waveguide core 18.

With reference to FIGS. 10, 11, 11A in which like reference numeralsrefer to like features in FIGS. 5, 6, 6A and in accordance withalternative embodiments, the distance, D1, between the side surface 42of the taper 38 and the side surface 44 of the taper 40 in the couplingregion 52 may decrease with decreasing distance from the waveguide core18 and the side surface 21 of the photodetector pad 20. In anembodiment, the distance, D1, between the side surface 42 of the taper38 and the side surface 44 of the taper 40 in the coupling region 52 maybe less than the distance, D2, between the side surfaces 42, 44 in thebody 36. Independent of this modification to the tapers 38, 40, width,W, of the tapered section 31 of the photodetector pad 20 decreases withincreasing distance from the waveguide core 18.

With reference to FIG. 12 in which like reference numerals refer to likefeatures in FIG. 7A and in accordance with alternative embodiments, thelight-absorbing layer 34 may be formed on the top surface 19 of thephotodetector pad 20 instead of being formed in the trench 28 in thephotodetector pad 20. The light-absorbing layer 34 is arranged fullyabove the photodetector pad 20 with the tapered section 31 of thephotodetector pad 20 positioned in a level that is fully below the levelof the tapers 38, 40 of the light-absorbing layer 34. Thelight-absorbing layer 34 may have a shape as shown in FIG. 5, FIG. 8, orFIG. 10.

With reference to FIG. 13 in which like reference numerals refer to likefeatures in FIG. 10 and in accordance with alternative embodiments, therespective tips 35 of the tapers 38, 40 may extend to intersect the sidesurface 21 of the photodetector pad 20. The modification may beaccomplished by changing the shape of the sections 32, 33 of the trench28 during the patterning of the photodetector pad 20. Independent ofthis modification to the tapers 38, 40, the width, W, of the taperedsection 31 of the photodetector pad 20 decreases with increasingdistance from the waveguide core 18. In the representative embodiment,the respective tips 35 of the tapers 38, 40 may extend to intersect aportion of the side surface 21 to which the waveguide core 18 iscoupled. The light-absorbing layer 34 may alternatively have a shape asshown in either FIG. 5 or FIG. 8 with tips 35 intersecting the sidesurface 21.

The methods as described above are used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (e.g., as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. Thechip may be integrated with other chips, discrete circuit elements,and/or other signal processing devices as part of either an intermediateproduct or an end product. The end product can be any product thatincludes integrated circuit chips, such as computer products having acentral processor or smartphones.

References herein to terms modified by language of approximation, suchas “about”, “approximately”, and “substantially”, are not to be limitedto the precise value specified. The language of approximation maycorrespond to the precision of an instrument used to measure the valueand, unless otherwise dependent on the precision of the instrument, mayindicate +/−10% of the stated value(s).

References herein to terms such as “vertical”, “horizontal”, etc. aremade by way of example, and not by way of limitation, to establish aframe of reference. The term “horizontal” as used herein is defined as aplane parallel to a conventional plane of a semiconductor substrate,regardless of its actual three-dimensional spatial orientation. Theterms “vertical” and “normal” refer to a direction perpendicular to thehorizontal, as just defined. The term “lateral” refers to a directionwithin the horizontal plane.

A feature “connected” or “coupled” to or with another feature may bedirectly connected or coupled to or with the other feature or, instead,one or more intervening features may be present. A feature may be“directly connected” or “directly coupled” to or with another feature ifintervening features are absent. A feature may be “indirectly connected”or “indirectly coupled” to or with another feature if at least oneintervening feature is present. A feature “on” or “contacting” anotherfeature may be directly on or in direct contact with the other featureor, instead, one or more intervening features may be present. A featuremay be “directly on” or in “direct contact” with another feature ifintervening features are absent. A feature may be “indirectly on” or in“indirect contact” with another feature if at least one interveningfeature is present.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration but are not intended tobe exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. A structure comprising: a waveguide core; and aphotodetector including a photodetector pad coupled to the waveguidecore and a light-absorbing layer coupled to the photodetector pad, thelight-absorbing layer including a body, a first taper that projectslaterally from the body toward the waveguide core, and a second taperthat projects laterally from the body toward the waveguide core, and thephotodetector pad including a tapered section that is laterallypositioned between the first taper and the second taper of thelight-absorbing layer.
 2. The structure of claim 1 wherein thephotodetector pad has a side surface, and the first taper and the secondtaper of the light-absorbing layer each have a respective tip that isspaced from the side surface by a gap.
 3. The structure of claim 2wherein the waveguide core is connected to the side surface, and thetapered section of the photodetector pad and the waveguide core arealigned along a longitudinal axis.
 4. The structure of claim 1 whereinthe photodetector pad has a side surface, and the first taper and thesecond taper of the light-absorbing layer each have a respective tipthat intersects the side surface.
 5. The structure of claim 4 whereinthe waveguide core is connected to the side surface, and the taperedsection of the photodetector pad and the waveguide core are alignedalong a longitudinal axis.
 6. The structure of claim 1 wherein thephotodetector pad and the waveguide core comprise single-crystalsilicon, and the light-absorbing layer comprises germanium.
 7. Thestructure of claim 1 wherein the photodetector pad includes an anode anda cathode, and the light-absorbing layer is laterally positioned betweenthe anode and the cathode.
 8. The structure of claim 1 wherein thephotodetector pad has a top surface, and the light-absorbing layer ispositioned on the top surface of the photodetector pad.
 9. The structureof claim 1 wherein the photodetector pad has a top surface and a trenchextending from the top surface partially through the photodetector pad,and the light-absorbing layer is positioned in the trench.
 10. Thestructure of claim 1 wherein the tapered section of the photodetectorpad has a width that decreases with increasing distance from thewaveguide core.
 11. The structure of claim 1 wherein the photodetectorpad has a side surface, the waveguide core is coupled to a portion ofthe side surface, and the tapered section of the photodetector pad has awidth that decreases with increasing distance from the side surface. 12.The structure of claim 1 wherein the photodetector pad has a sidesurface, the waveguide core is coupled to a portion of the side surface,the first taper has a first side surface, the second taper has a secondside surface, and the first side surface of the first taper is laterallyseparated from the second side surface of the second taper by a distancethat increases with decreasing distance from the side surface of thephotodetector pad.
 13. The structure of claim 12 wherein the taperedsection of the photodetector pad has a width that decreases withincreasing distance from the side surface.
 14. The structure of claim 1wherein the photodetector pad has a side surface, the waveguide core iscoupled to a portion of the side surface, the first taper has a firstside surface, the second taper has a second side surface, and the firstside surface of the first taper is laterally separated from the secondside surface of the second taper by a distance that is constant withdecreasing distance from the side surface of the photodetector pad. 15.The structure of claim 14 wherein the tapered section of thephotodetector pad has a width that decreases with increasing distancefrom the side surface.
 16. The structure of claim 1 wherein thephotodetector pad has a side surface, the waveguide core is coupled to aportion of the side surface, the first taper has a first side surface,the second taper has a second side surface, and the first side surfaceof the first taper is laterally separated from the second side surfaceof the second taper by a distance that decreases with decreasingdistance from the side surface of the photodetector pad.
 17. Thestructure of claim 16 wherein the tapered section of the photodetectorpad has a width that decreases with increasing distance from the sidesurface.
 18. A method comprising: forming a waveguide core and aphotodetector pad coupled to the waveguide core; and forming alight-absorbing layer coupled to the photodetector pad, wherein thelight-absorbing layer includes a body, a first taper that projectslaterally from the body toward the waveguide core, and a second taperthat projects laterally from the body toward the waveguide core, thephotodetector pad includes a tapered section that is laterallypositioned between the first taper and the second taper of thelight-absorbing layer, and the light-absorbing layer and thephotodetector pad are included in a photodetector.
 19. The method ofclaim 18 wherein the photodetector pad has a side surface, the waveguidecore is connected to the side surface, and the first taper and thesecond taper of the light-absorbing layer each have a respective tipthat is either spaced from the side surface by a gap or intersects theside surface.
 20. The method of claim 18 wherein the photodetector padhas a side surface, the waveguide core is coupled to a portion of theside surface, and the tapered section of the photodetector pad has awidth that decreases with increasing distance from the side surface.